The Study Of Spatial Variability Of Soil Heavy Metals At Different Scales Based On GIS And Geo-statistics

In recent years,with the continuous development of human industrial activities,the mining activities has been intensified,the improper handle of waste residues and the exhaust gas have caused the mine and factories near area heavy metal content in the soil exceed the standard,causing heavy metal pollution in the soil and affecting human health.In this study,we take the downwind direction of the east of the realgar mine in Shimen County,Hunan Province as the research object,based on 170 soil samples collected by the National Key Research and Development Project?2018YFC1802604??including 43 town scales,85 village scales,and 42 field scales?,using geo-statistics and GIS combined data processing and analysis methods to study the spatial variability of soil heavy metals at three different scales in towns,villages and fields,identify the soil heavy metal over-standard elements at different scales in the study area and analyze their spatial variability,distribution characteristics,pollution sources and pollution status,visualization of the spatial variability of heavy metals and pollution status,provides the basis decision-making for environmental governance and planning of relevant government departments.The main conclusions are as follows:1)Seven heavy metals were analyzed and tested in the research area,the heavy metal elements As and Cd at the sampling points in the research area were found to exceed the standard,a few sampling points of the elements Pb,Cr,Zn,Cu,and Ni exceeded the national secondary standard,it was verified that the point was located near the mine refinery.The correlation analysis and principal component analysis results that two scales As,Pb,Cu,and Zn in the study area soil may mainly came from the same pollution source-the local realgar refinery;The Cd element has little correlation with other pollution elements,and is evenly distributed in the study area,which may mainly came from human activities,such as the use of pesticides and fertilizers.2)From the results of the Moran's I coefficient,nugget value C₀,and range A₀ of the heavy metal elements As and Cd at the town and village scales,it can be known that at the town scale,it is helpful to reveal the range of soil heavy metals affected by random factors?natural and human factors?,and to understand the micro-domain structural characteristics of soil heavy metals;It is helpful to reveal the size of soil heavy metals affected by structural factors at the village level,understand the effect level of terrain and parent material.With the reduction of the research scale,the spatial autocorrelation distance of the heavy metal As and Cd element content increases,and the effects of large-scale structural factors such as terrain,parent material,and land use factors on As and Cd content gradually increase,while the influence of small-scale structural factors,random factors has gradually weakened.3)In the two scales,the model determination coefficients of As are 0.73 and 0.99,and the model determination coefficients of Cd are 0.63 and 0.98 respectively,the accuracy of the model is high.Kriging interpolation results of element As and Cd at the two scales are as follows:the village-scale As,Cd element second level and third level pollution areas have decreased,at both scales,the As,Cd element first level pollution areas are in the northeast direction of the study area,and the third level pollution areas are in the southwest direction of the study area is mainly because the area is a realgar mine,and close to the refinery,it is manifested as coordinated pollution of As and Cd.4)The uneven distribution of As and Cd elements at the field scale is mainly caused by external factors.The content of element As and Cd in different fields is quite different,and the content of element As and Cd is also different in the same field.The coefficient variation of heavy metals between fields is between 10%and 100%,which belongs to medium intensity variation.The index evaluation results showed that the sampling points of field A and C of the As elemental were seriously exceeded and reached severe pollution;The field D and E contents of the Cd elemental were higher than those of A and C,and some areas reached moderate pollution.The Kriging interpolation results on the field scale indicate that the heavy metal content in the field near the highway and residential areas is higher,which indicates that the field pollution is greatly disturbed by human factors.5)In this paper we use the Nemerow comprehensive index,geo-accumulation index,pollution load index and potential ecological risk index to evaluate the study area for consider the scope of the study area,the types of heavy metal pollution,and the geographical environment,four evaluation methods have different results,the geo-accumulation index and pollution load index evaluate area are larger than the area of Nemerow index,the different reference standards and emphasis of evaluation methods may be the reason of the inconsistent evaluation results of heavy metal pollution.The study area is relatively polluted by human factors,the area caused by human factors at the town level is 9.33 km²,accounting for 66.31%of the total area,the area polluted by human factors at the village level is 10.30 km²,which accounts for 73.21%.Potential ecological hazard index indicates that the potential ecological hazards of the heavy metals in the soil of study area are arranged in order from Cd>As>Cu>Pb>Ni>Cr>Zn,among them,Cd is the most important contributing factor of ecological risk,ecological hazards in the study area are more serious with As and Cd.